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If asteroids got dizzy

Space rocks on the spin cycle: These images show Beta, the smaller piece of asteroid 1999 KW4, as it makes its way around the larger piece, Alpha. Both pieces are also spinning on their own axes.

Then near-Earth asteroid 1999 KW4 would be about to lose its lunch right now. Scientists have discovered that the main piece of this asteroid, whose name is "Alpha," is spinning so fast that it would break apart if it went any faster. Alpha also has a little buddy named "Beta." Beta is revolving around its own axis, but it also circles around Alpha. That's a lot of spinning!

Together, Alpha and little Beta make up a "binary" asteroid. So far, 840 large asteroids have been discovered floating in the inner solar system, near enough to our planet to be called -- you guessed it -- "near-Earth asteroids." Of these, 28 are binaries, in which one object rotates around another.

Binary systems in general are common in space, including binary stars and planet-moon pairs. Binary asteroids like 1999 KW4 are the smallest and closest examples, so they are the easiest types of binary systems to study.

When 1999 KW4 swooped by Earth in May 2001, Steven Ostro of the Jet Propulsion Laboratory at the California Institute of Technology and his colleagues got a look at both its pieces using a technique called radar mapping. This involves bouncing radar off the surface of a faraway object and measuring the radar's strength when it returns, as well as how long it takes.

The scientists learned that Alpha is made up of loosely-packed rubble and completes a spin around its axis every 2.8 hours. Spinning much faster than that would probably cause the rubble to break up and spray out into space. Beta is more oval-shaped and denser than Alpha.

The researchers think that a close encounter with Earth or the Sun may have been what sent the asteroid spinning in the first place.

These findings will be published online by the journal Science, at the Science Express website, on 12 October.